Skylights, good for daylighting, bad for energy efficiency
Generally, we may prefer skylight over windows as they introduce more daylight inside the house. More daylit houses demand less artificial lighting and hence, less cooling demand. However, skylight contribution to the reduction in cooling demand is far offset by its role in allowing high level of solar heat gains inside. We may balance the two mentioned impacts, by installing a tinted-glazed skylight, however, energy efficiency still lacks behind the limited daylighting. The reason is roofs play a major role in blocking heat transfer rates in winter, Hence we tend to add high levels of insulation to the roofs. However, adding a skylight with even a low U-value (say U2.0) is still 10 times more than that of a roof having an NCC minimum insulation requirement (for climate zone 6, a Total R-value of R5.1 results in U0.2). Hence, skylights decrease building thermal performance in both summer and winter.
Thermal bridging effect is another reason why skylights are bad for energy compliance. Thermal bridge simply means a pass of least thermal resistance for heat transfer in a wider area of material with a high thermal resistance. In this case, a roof with a Total R-value of R5.1 (U-value of 0.2) has a very high thermal resistance, however, installing a skylight over with a U-Value of 2.0 creates an opening in an insulated roof and creates a thermal bridge or cold bridge. To avoid thermal bridging effect, NCC doesn’t allow ceiling penetration of more than 0.5% of the ceiling area. A ceiling with more than 0.5% penetration must be insulated even more (more than the NCC minimum requirement) to compensate for the loss of insulation (NCC Vol 2.0 Table 126.96.36.199h).